While linear motor based compressors and pumps offer certain theoretical advantages such as mechanical simplicity and reduced friction relative to traditional reciprocating machines, there are numerous design challenges unique to linear compressors which must be addressed in order to make them a practical alternative in the general market. U.S. Pat. No. 4,965,864 by Roth and Roth addresses key issues such as magnetic circuitry and control logic which are applicable to a wide range of motor, pump and compressor applications. Linear motors of this type are properly referred to as tubular motors in that the drive coils are arranged so as to form a cylinder or tube through which the stator is axially driven. Further research has led to designs particularly well adapted to compressor applications, especially those in which flow demand and pressure ratios vary widely and independently as they do in air conditioning and refrigeration systems. Most linear compressors now available rely on a single phase electrical design which produces force only through a limited distance and in a single direction with return force provided by a mechanical spring. Examples of this design include U.S. Pat. No. 5,342,176 to Redlich and 5,261,799 to Laskaris. They do not scale well above fractional horsepower sizes and speed is fixed by mechanical resonance. These linear compressors show promise for household refrigerator applications but may not be suitable for air conditioning applications. Traditionally, air conditioning compressors have been designed so that their narrow peak efficiency curve matches the expected peak pressure and flow requirements for the system in which they are to be installed. This leads to a situation in which manufacturers find it necessary to offer a large number of nearly energy loss because the compressor will spend the majority of it's operating hours working well below capacity in the lower regions of it's efficiency curve. At the same time, if peak design conditions are exceeded the compressor may fail catastrophically. For this reason systems are often oversized, further reducing their efficiency. Some of the highest efficiency residential and light commercial air conditioning systems available today rely on two compressors in parallel which can be brought on line independently as conditions warrant. As a result these systems are bulky, complex, and expensive.
While oil free operation is beneficial in an air conditioning compressor, in some applications, such as oxygen or medical compressed air, it is required. While linear compressors have a sliding piston, the transverse force created in the process of changing the rotary motion of the motor into the reciprocating motion required by the piston has been eliminated. Early models of linear compressor have demonstrated reliable oil free operation. Other applications for pumps and compressors require metering of the working fluid. This is typically done with external sensors, but with a positive displacement pump or compressor it can be done internally by monitoring strokes.